Workers that work at heights where there is a risk of injury from falling are required by law (Occupational Health and Safety Legislation) to calculate their Free Fall Distance (FFD) and Clearance Required (CR), and keep them within safe parameters. There is a long series of mathematical calculations necessary to determine the FFD and CR depending on the equipment the workers are using and their work site configuration. Workers in these situations often have limited education and mathematical skills, which make it challenging for them to accurately calculate the FFD and CR, as required by law.
There are website applications available online that will permit a worker to calculate FFD and CR. However, the equipment required to use these applications (e.g., computers, tablets or mobile telephones) is not intrinsically safe and is often unfeasible or prohibited from worksites.
Table 1 is a list of acronyms that are used in industry to determine FFD and CR. FIG. 10 is an illustration which assists in the understanding of the terms in the following TABLE 1.
TABLE 1AcronymDescriptionLLength of lanyard, shock absorber & connecting hardware(initial overall length)BDistance from the feet to the harness D-Ring (Standingworker)CDistance from the feet to the anchorage level (Standingworker)SAEShock Absorber ExtensionDSD-ring Slippage (typically 1 ft or 0.3 m)SMSafety Margin between bottom of feet and level below(typically 2 ft (0.6 m) or 3 ft (0.9 m))FFDFree Fall Distance = L + B − CTFDTotal Fall Distance = FFD + DS + SAECRClearance RequiredCRAClearance Required from the Anchor = TFD + SM + C
Exposure to arrest forces that exceed safe limits can lead to injuries, such as: ruptured intestines, fractured liver, traumatic abdominal organ displacement into the chest cavity, and or fractures and tears to the extremities. Workers utilizing fall protection equipment must ensure that if they fall, the arrest forces imposed on their bodies during fall arrest are kept within safe limits. These arrest forces are determined in part by their FFD. As the free fall distance increases, so does the impact force on their bodies, anchor or anchorage, and all system components which could fail if forces are exceeded.
Free fall distance (FFD) is defined as the vertical distance from the onset of a fall to the point where the fall-arrest system begins to apply force to arrest the fall. To calculate the FFD, the length of lanyard, shock absorber and connecting hardware (L) is added to the distance from the worker's feet to the harness D-Ring (B), which is subtracted from the distance between the working level and anchor height (C). The formula may be stated as Free Fall Distance=Length of lanyard+D-Ring Height−Anchor Height and may be stated as follows:FFD=L+B−C  (1)
Total fall distance (TFD) is defined as the maximum distance fallen by the worker using a fall-arrest system between the onset of a fall and the instant when the worker first achieves zero vertical velocity. Total-fall distance is often determined as the displacement of the dorsal D-Ring on the full-body harness and is the sum of the free fall and the deceleration distance. It also includes any applicable swing-fall distance. It is essentially the vertical distance from the point at which a worker falls to the point where the fall stops (that is, after all fall arresting system components have extended).
To calculate the TFD, the Free Fall Distance (FFD) is added to the D-Ring Slippage (DS) and the extension of the shock absorber (SAE). The maximum shock absorber extension (as indicated on the unit) must be used when determining the total fall distance. The formula may be stated as Total Fall Distance=Free Fall Distance+D-Ring Slippage+Shock Absorber Extension and may be written as follows:TFD=FFD+DS+SAE  (2)
CSA Standard Z259.16-04 Design of Active Fall Protection Systems defines clearance as: “the distance from a specified reference point, such as the working platform or anchorage of a fall-arrest system, to the highest obstruction that a worker might encounter during a fall.”
Minimum fall clearance (CR) from the Anchor is the total fall distance (TFD) plus a safety margin (SM) of at least 0.6 m (2 ft) plus Anchor height. The formula may thus be written as follows:CRA=TFD+SM+C  (3)
In summary, without the present invention, the worker must remember the following formulas (in bold) and complete calculations (like the example calculations shown below), which leave room for error and may lead to worker injury if miscalculated:FFD=L+B−C  (1)FFD=6+5−7FFD=4TFD=FFD+DS+SAE  (2)TFD=4+1+4TFD=9CRA=TFD+SM+C  (3)CRA=9+2+7CRA=18
It would be beneficial if a system was developed which did not require a worker to remember the aforementioned formulas or perform calculations similar to those shown above, so as to reduce errors and worker injuries.